Langmuir 1996, 12, 6681-6690
6681
Confinement-Induced Morphological Changes in Diblock Copolymer Films Nagraj Koneripalli, Rastislav Levicky, and Frank S. Bates* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
John Ankner and Helmut Kaiser Missouri University Research Reactor, Columbia, Missouri 65211
Sushil K. Satija National Institute of Standards and Technology, Reactor Radiation Division, Gaithersburg, Maryland 20899 Received June 20, 1996. In Final Form: September 25, 1996X We have studied the ordering behavior of a (diblock copolymer, poly(styrene-d8)-poly(2-vinylpyridine) (dPS-PVP), in a confined geometry by neutron reflectivity, transmission electron microscopy, and atomic force microscopy. The diblock copolymer was confined between a silicon substrate (Si) on one side and a glassy polymer, poly(2-methylvinylcyclohexane) (P2MVCH), on the other side. In such a geometry, incompatibility between the natural domain period of the diblock copolymer (D*) and the film thickness (L) creates frustration that can be varied by controlling the copolymer film thickness. As the degree of frustration is increased (i.e., film thickness is decreased), the domain periods of the lamellae become progressively distorted from D*, and the lamellae orient with dPS/PVP interfaces parallel to the confining surfaces. The dPS block wets the P2MVCH confining wall and the PVP block wets the Si substrate. There is a limit, however, to the extent of distortion of the lamellar domain period; a further increase in frustration results in a sharp transition to a complex layered morphology that has a heterogeneous in-plane structure adjacent to the P2MVCH confining wall. In this morphology, both the dPS and PVP are located near the P2MVCH confining wall and only PVP is located at the Si confining wall. The sharp transition in the morphology is interpreted in the context of competing surface and bulk interactions. By removal of one of the confining walls, the frustration is relieved and a lamellar structure parallel to the surfaces is recovered with a domain period of D*.
I. Introduction Block copolymers embrace many of the unique properties associated with polymer macromolecules and small molecule amphiphiles. These materials possess the mechanical and flow properties (processibility) of polymer macromolecules and can self-assemble into a wide array of microstructures. Self-assembly is synonymous with interfacial activity, and a fundamental understanding of this behavior has been a subject of considerable interest.1-3 The geometrically periodic microstructures in self-assembling molecules occur on length scales that are closely related to their molecular dimensions. In thin films, incompatibility between this length scale and the film thickness creates frustration, the effects of which can be probed in a confined geometry. Physical confinement provides a classic route to the two-dimensional limit of materials. A natural consequence of this reduced dimensionality is a greatly enhanced contribution from surface interactions to the total free energy. A delicate balance between the surface and bulk interactions produces profound effects on properties such as glass transition,4 wetting,5,6 rheological response,7,8 and phase be* Author for correspondence. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) Bates, F. S.; Fredrickson G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. (2) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 597. (3) For a recent review, see: Krausch, G. Mater. Sci Eng. 1995, Vol. 14. (4) Fehr, T.; Lowen, H Phy. Rev. E 1995, 52, 4016. (5) Tanaka, H. Phys. Rev. Lett.1993, 70, 53.
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havior9,10 in a host of materials. In this report, we focus on the structural aspects (ordering) of diblock copolymers in a confined geometry. External surfaces and interfaces influence the nature of ordering in block copolymers3,11-13 and, in general, introduce a directionality to ordering. For instance, in thin films of symmetric A-B diblock copolymers, a preferential affinity of the blocks to the two surfaces causes a lamellar ordering with A-B interfaces parallel to the surfaces. The chain connectivity and the bulk interactions constrain the ordering on a fixed length scale (domain period) D*. These two restrictions, together with the film incompressibility, lead to terracing on the free surface of the film if the copolymer film thickness is incommensurate with D*. Terrace formation occurs so that the film thicknesses are quantized in integer multiples of domain period, (nD*), when the same block segregates to both surfaces, and half-integer multiples of domain period, (n + 1/2)D*, when different blocks segregate to the two surfaces. These effects have been well documented in the literature.14-16 Terrace formation is a mechanism for the copolymer to relax to its natural domain period, D*, while (6) Tang, J. Z.; Harris, J. G. J. Chem. Phys. 1995, 103, 8201. (7) Mel’nichenko, Yu. B. et al. J. Chem. Phys. 1995, 103, 2016. (8) Klein, J.; Kumacheva, E. Science 1995, 269, 816. (9) Dadmun, M. D.; Muthukumar, M. J. Chem. Phys. 1994, 101, 10038. (10) Karaborni, S. Phys. Rev. Lett. 1994, 73, 1668. (11) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. Phys. Rev. Lett. 1989, 62, 1852. (12) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. J. Chem. Phys. 1990, 92, 5677. (13) Foster, M. D.; Sikka, M.; Singh, N.; Bates, F. S.; Satija, S. K.; Majkrzak, C. F. J. Chem. Phys. 1992, 96, 8605.
© 1996 American Chemical Society
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preserving a lamellar layering parallel to the surfaces. This mechanism, however, is not accessible when the copolymer is confined between hard walls. In thin films of confined diblock copolymer, two competing phenomena, a tendency to preserve the natural domain period D* (governed by bulk interactions) and a tendency for the blocks to wet the two confining hard walls, can impose severe configurational restrictions on the chains. This can produce changes in the morphology and/or wetting characteristics. Diblock copolymer confinement has been studied theoretically through self-consistent field calculations,17 phenomenological models,18,19 and Monte Carlo calculations.20-22 Turner18 has obtained analytical expressions for the free energy of a copolymer confined between flat plates that is constrained to order with interfaces parallel to the surfaces. A perturbation of domain period as the plate separation is changed is predicted by this theory. Walton et al.19 extended Turner’s theory by relaxing the constraint of ordering occurring parallel to the surfaces. They find that a perpendicular lamellae configuration is favored over a limited range of film thickness. Kikuchi and Binder20,21 studied this problem by Monte Carlo simulations. For compatibility between the plate separation distance and D* of the copolymer, they observed a lamellar ordering parallel to the confining surfaces. In other situations, however, they find a “tilted lamellar” structure, or multiple coexisting lamellar orientations. Monte Carlo methods were also used by Brown and Chakrabarti22 to study this problem. They found the configuration of lamellae to be either parallel or perpendicular to the confining surfaces depending on the separation distance between the hard walls. Qualitatively, the essence of the models and calculations in refs 19-22 is the prediction of a change in the configuration of lamellae from a parallel to a perpendicular orientation with respect to the confining surfaces, as the degree of frustration is increased. Techniques for experimentally studying this problem have been developed by us and others.23,24 Confinement effects of poly(styrene)-poly(methyl methacrylate) (PSPMMA) between similar hard walls and poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) between similar and dissimilar hard walls have been reported.23,24 In both copolymer systems, a “saw-tooth” variation in the domain period about D* is observed as the film thickness of the copolymer is varied, indicative of both a stretched and compressed chain configuration. Lamellae are found to orient only parallel to the confining surfaces, and frustration is relieved by perturbing the domain period. This is a consequence of relatively large film thicknesses studied that limit the maximum frustration attainable. Coupled with the strong surface effects (enthalpic23 and entropic24), this precludes changes in the wetting behavior or a configuration other than lamellar orientation parallel to the surfaces. Confinement between surfaces composed (14) Coulon, G.; Collin, B.; Ausserre, D.; Chatenay, D.; Russell, T. P. J. Phys. (Paris) 1990, 51, 2801. (15) Coulon, G.; Collin, B.; Chatenay, D.; Gallot, Y. J. Phys. II 1993, 3, 697. (16) Bassereau, P.; Brodbreck, D.; Russell, T. P.; Brown, H. R.; Shull. K. R. Phys. Rev. Lett. 1993, 71, 1716. (17) Shull, K. R. Macromolecules 1992, 25, 2122. (18) Turner, M. S. Phys. Rev. Lett. 1992, 69, 1788. (19) Walton, D. G.; Kellogg, G. J.; Mayes, A. M.; Lambooy, P.; Russell, T. P. Macromolecules 1994, 27, 6225. (20) Kikuchi, M.; Binder, K. Europhys. Lett. 1993, 21, 427. (21) Kikuchi, M.; Binder, K. J. Chem. Phys.1994, 101, 3367. (22) Brown G.; Chakrabarti, A. J. Chem. Phys. 1995, 102, 1440. (23) Lambooy, P.; Russell, T. P.; Kellogg, G. J.; Mayes, A. M.; Gallagher, P. D.; Satija, S. K. Phys. Rev. Lett. 1994, 72, 2899. (24) Koneripalli, N.; Singh, N.; Levicky, R.; Bates, F. S.; Gallagher, P. D.; Satija, S. K. Macromolecules 1995, 28, 2897.
Koneripalli et al.
Figure 1. Chemical structure of dPS-PVP diblock copolymer and the P2MVCH polymer (confining wall).
of a random copolymer has induced both parallel and perpendicular lamellar orientations.25 In this article, we examine the ordering characteristics of a diblock copolymer in confinement as a function of its thickness under conditions where the surface energy contributions and the energetic penalty arising from distortion of the domain spacing are comparable. In such a regime, a transition in the morphology is anticipated from one that is dominated by the surface energetics to one that is dominated by the bulk interactions. Accessibility of such a regime is achieved by suitable experimental design including investigation of copolymer film thicknesses in the vicinity of one bilayer dimension (to augment the severity of configurational frustration). The copolymer film thickness is progressively varied and the ordering of the copolymer is quantitatively examined by neutron reflectivity. Following the measurements on the confined samples, the confining wall is stripped (see below), and the structure of the copolymer is re-examined. The morphology seen in the same copolymer sample with and without the presence of the hard wall affords a comparison of the structural changes and accentuates the effects of confinement. The neutron reflection results are complemented and supplemented by characterization of the inplane structure by transmission electron microscopy (TEM) and surface topology by atomic force microscopy (AFM) and the surface properties by X-ray photoemission spectroscopy (XPS) and contact angle measurements. II. Experimental Section The copolymer used in this study was a poly(styrene-d8)poly(2-vinylpyridine) (dPS-PVP) (Figure 1) diblock that was synthesized by anionic polymerization as described elsewhere.26 It had a number average molecular weight Mn ) 16 300 g/mol with a polystyrene block volume fraction of 0.5 and a polydispersity index Mw/Mn of 1.05. This copolymer was confined between an oxide-stripped silicon wafer on one side and poly(2-methylvinylcyclohexane) (P2MVCH), a glassy homopolymer, on the other side. P2MVCH was synthesized in a two-step process. 2-Methyl styrene was polymerized anionically, initiated by sec-butyllithium.27 The resulting polymer, poly(2-methylstyrene), was subsequently catalytically hydrogenated to remove all (>99%) unsaturation. The number average molecular weight of the P2MVCH was 425 000 g/mol with a polydispersity index of 1.06 and a glass transition temperature of 186 °C.27 Thin film samples were spun on 10 cm diameter silicon wafers of 0.476 cm thickness having a (111) orientation. Silicon wafers were cleaned in a 30%-70% mixture of hydrogen peroxide and sulfuric acid at 120 °C for 10 min. The native oxide was etched in a 10% hydrofluoric acid bath (90 s treatment). The wafers were then rinsed in deionized water, cleaned with high-purity pressurized nitrogen gas and immediately used for spin coating. Solutions of dPS-PVP copolymer in toluene in the concentration range of 0.3-0.9 wt % were filtered with a 0.45 µm filter and then used to make films of different thicknesses by suitably changing the spinning speeds. The films were dried under ambient conditions, and the copolymer film thickness was (25) Kellogg, G. J.; Walton, D. G.; Mayes, A. M.; Lambooy, P.; Russell, T. P.; Gallagher, P. D.; Satija, S. K. Phys. Rev. Lett. 1996, 76, 2503. (26) Schulz, M. F.; Khandpur, A. K.; Bates, F. S.; Almdal, K.; Mortensen, K.; Hajduk, D. A.; Gruner, S. M. Macromolecules 1996, 29, 2857. (27) Gehlsen, M. D.; Weimann, P. A.; Bates, F. S.; Harville, S.; Mays, J. W.; Wignall, G. D. J. Polym. Sci.1995, 33, 1527.
Morphology of Confined Diblock Copolymer Films measured by X-ray reflectivity and ellipsometry (see below). The confining wall was deposited by spin coating P2MVCH on top of the copolymer from a filtered solution of n-heptane. n-Heptane is a nonsolvent for the copolymer, and spin coating P2MVCH from a solution of n-heptane does not affect the copolymer film; this was inferred from the consistency in the copolymer film thickness measurements from X-ray reflectivity and ellipsometry on a test sample after repeated washing with pure n-heptane (
